1. Field of the Invention
The present invention relates generally to electroerosion machining, and more particularly, to quickly forming small diameter holes completely through a conductive workpiece using electroerosion.
2. Description of the Prior Art
U.S. Pat. No. 4,393,292, entitled "Method Of And Apparatus For Electrical Discharge Machining A Small And Deep Hole Into Or Through A Workpiece" that issued Jul. 12, 1983, on a patent application filed in the name of Kiyoshi Inoue ("the Inoue '292 Patent"), discloses that electroerosion (a/k/a electrical discharge machining or EDM) is capable of drilling holes having a diameter of 1.0 mm or less, i.e. 0.040 inches or less, and a depth-to-diameter ratio of at least 5 into or through an electrically conductive workpiece using a tubularly shaped electrode at speeds of 20 to 30 mm/min, i.e. a speed of approximately 1 inch per minute. To achieve such rapid machining, the Inoue '292 Patent discloses supplying a deionized water machining fluid into an electroerosion machining gap between the tubular electrode and the workpiece. The Inoue Patent further discloses that this water machining fluid preferably has a specific resistivity in excess of 10.sup.4 ohm-cm, and must be supplied to the electrode at a pressure of at least 20 kg/cm.sup.2 ; preferably at 25 kg/cm.sup.2 ; and more desirably at 40 kg/cm.sup.2. The applicant hereby incorporates by reference the Inoue '292 Patent.
Since the filing of the application which issued as the Inoue '292 Patent, further refinements have been developed in the electroerosion technique which it discloses. For example, in an effort to increase machining speed for larger diameter tubular electrodes at a high power setting, 10% to 50% of various different water soluble glycol-based materials have been added to deionized water to increase the machining fluid's boiling point and dielectric effect. Alternatively, in an effort to reduce wear of the tubular electrode during machining, 5% to 10% of graphite has been added to the deionized water machining fluid. Moreover, recent advances in small hole drilling electroerosion equipment now permit forming holes into or through a conductive workpiece with a depth-to-diameter ratio of up to 500:1. For example, using a 1.0 mm diameter tubular electrode is now possible to drill a hole through a one-half meter thick workpiece, i.e. drilling a 0.040 inch diameter hole through a 20.00 inch thick workpiece. Furthermore, it has also been found possible to form holes using tubular electrodes with a diameter down to 0.1 mm and up to 6.5 mm while maintaining a high machining speed.
Despite the remarkable machining performance achieved by the electroerosion technique disclosed in the Inoue '292 Patent, drilling such holes becomes unstable as the tubular electrode pierces through the workpiece. Thus, as a tubular electrode begins to pierce through the workpiece, machining speed slows drastically, and in some instances the electrode may even weld to the workpiece. Frequently, the time required to finally form an exit hole completely through the workpiece may equal or even exceed the time required to drill through the workpiece prior to forming the exit hole. For example, drilling a hole through a 1.0 inch thick D2 steel workpiece using a 3.0 mm diameter electrode requires approximately 60 seconds until the tubular electrode initially begins forming an opening through the workpiece. However, completing the exit hole through the workpiece may also require as long as 60 seconds. Consequently, drilling holes through workpieces using the electroerosion technique disclosed in the Inoue '292 Patent could be facilitated and the overall productivity achieved in electroerosively drilling small and deep holes could effectively be doubled if it were possible to stabilize machining and maintain the technique's high machining speed while finally piercing through a workpiece.